1. Field of the Invention
The present invention relates generally to fluid dispensing pipettes and, more particularly, to a unitary volumetric pipette formed from separate injection molded components joined together to create a precision fluid delivery device which is capable of dispensing an extremely accurate amount of liquid. The invention is also directed to a method for making this unitary volumetric pipette.
2. Description of the Prior Art
Pipettes are well-known devices which are designed to dispense measured quantities of liquids, particularly in uniform drops of a given volume. Pipettes have had widespread usage in a number of industries where accurate measurement and delivery of fluids are required, particularly the medical and laboratory testing and analysis fields.
Numerous types of pipettes have been proposed and developed throughout the years. Most rely on a construction which includes a narrow tube or stem into which the liquid is drawn. A flexible bulb, usually made from rubber or a similar flexible material, is connected to the stem to produce a vacuum when the bulb is squeezed to draw the liquid into the stem. Once fluid is drawn into the stem, it will remain there until the bulb is again squeezed by the user to release some or all of the fluid. By carefully manipulating the bulb, a user can generally release the fluid a drop at a time. The size or volume of the drop is usually determined by the size of the opening formed at the tip of the stem. The stem may also include calibrations which allow the user to deliver larger measured amounts of liquid at one time.
Early style pipettes were generally made from glass tubing and a flexible rubber bulb attached at one end of the tubing. Advances in plastic forming techniques have resulted in the development of disposable plastic transfer pipettes which prove to be somewhat reliable and eliminate a number of the disadvantages associated with glass pipettes. For example, glass pipettes are susceptible to breakage during transportation from the manufacturer and also during usage which can prove to be detrimental from a cost standpoint. Additionally, there is a potential for contamination in glass pipettes especially if the rubber bulb is interchanged on a number of glass tubings. Also, if the glass tubing is broken, the user is subjected to possible infection or contamination if cut by the broken glass.
Numerous methods for forming distinctive plastic transfer pipettes have thus been developed. For example, in U.S. Pat. No. Re. 31,555 to Garren, a method for creating a plastic pipette consists of a blow molding process in which a heat softened tubing is placed within a mold and expanded by air pressure to force the tubing against the mold to take on its particular shape. Other methods for forming other distinct plastic pipettes are disclosed in U.S. Pat. No. 4,779,768 to St. Amand and U.S. Pat. No. 4,563,104 to St. Amand which also utilize blow molding as the preferred method for forming the pipette. Blow molding, in fact, has been primarily used in the pipette industry since it provides a quick and relatively inexpensive process for mass producing disposable transfer pipettes.
The plastic disposable pipette is formed as a single piece which includes a stem portion and a built-in bulb for drawing the liquid into the stem. This disposable type of pipette has generally been utilized in the industry as a transfer pipette for transferring liquids which do not have to be precisely measured. Generally, a pipette can be classified as either a volumetric pipette or transfer pipette depending upon the accuracy of the delivery of the device. Volumetric pipettes are capable of delivering a much more precise amount of liquid than a simple transfer pipette and, as a result, require more precision in creating the stem portion which defines the chamber which retains and delivers the liquid. For the most part, volumetric pipettes are either manufactured from glass or injection molded plastic which forms the stem portion. The stem of a volumetric pipette is usually open at both ends and thus requires a pipette aid, such as a rubber bulb or similar device, to help draw up the liquid into the stem. Some volumetric pipettes are designed to allow the user to draw the liquid into the pipette through mouth suction. These type of volumetric pipettes can be unsafe since fumes or the liquid itself can be drawn into the user's mouth during usage. Additionally, these types of pipettes are often more expensive to manufacture than the disposable plastic transfer pipette.
The plastic blow molded transfer pipette elminates some of the disadvantages associated with volumetric pipettes since the bulb and stem are formed as a one-piece unit. As a result, there is less chance for the plastic pipette to become contaminated since the bulb portion is incapable of being separated from the stem and thus cannot be interchanged from stem to stem. As mentioned above, the bulb portion, if not permanently affixed to the stem portion, can be interchanged with various stems and can carry with it unwanted fumes or residual liquid from previous uses. Additionally, the built-in bulb portion eliminates the need to use mouth suction to draw the fluid into the stem. This eliminates any potential health risk to the user. Also, since the blow molded pipette is made from a plastic material which can be manufactured quite inexpensively, it is readily disposable and can be utilized for one time usage only. Thus the plastic blow molded transfer pipette has certain distinct advantages over the conventional volumetric pipettes.
While blow molding is the preferred method for producing the disposable plastic transfer pipette, there are serious drawbacks with this particular method which results in a pipette which is unable to deliver an extremely precise amount of liquid. This is attributable to the fact that the inner surface of the pipette, which defines the chamber that holds the fluid, is susceptible to defects and uneven formation during the blow molding process which directly impairs its ability to retain the amount of liquid it is designed to deliver. It should be appreciated that the critical control surface within any pipette, whether it be made from glass or plastic, is the inner surface since this particular surface defines the boundaries of the fluid chamber that stores and measures the fluid. For example, fluctuations in the wall thickness of the stem portion will effect the amount of fluid that can be stored in the fluid chamber and can provide an inaccurate delivery of fluid.
When blow molding methods are employed, air or a gas is forced into the interior of a heat softened tubing which causes the tubing to expand against the surface of the mold to retain its shape. If the air or gas pressure fluctuates within the mold, the wall thickness in the stem portion can vary causing either a larger or smaller fluid chamber to be created in the pipette. Any fluctuations in the wall thickness is detrimental since the pipette will no longer deliver the expected amount of liquid. In some instances, the fluctuation can be as high as ten percent which results in the pipette delivering a less than precise amount of liquid. In some industries, an imprecise delivery of fluid is detrimental and can cause erroneous test results. For this reason, plastic pipettes are generally used for bulk transfer of liquids when precision measurement and delivery are not required.
The formation of the opening at the tip of the stem of a plastic transfer pipette is also susceptible during the blow molding process since the tip can also be irregularly formed. As a result, the precision of the drop size delivered by the pipette can also be effected and can result in an inaccurate delivery of fluid.
As a result of the disadvantages associated with plastic transfer pipettes that are formed by a blow molding process, there is a need for an improved plastic pipette which can provide a more precise and accurate delivery of fluid and yet can still be manufactured inexpensively and quickly. Therefore, there is a need for a pipette and a method for making the same which will eliminate the above-noted disadvantages associated with disposable one-piece plastic pipettes which are now manufactured by blow molding processes.